EARLY DEVELOPMENT IN THE FROG. 357 



(6) By differential inhibition, and 



(c) By differential acclimation and recovery. 



3. The two modifications of the susceptibility method de- 

 scribed in this paper, (a) and (&) above, agree in showing that: 



(a) Those parts of the egg where development first begins and 

 proceeds most rapidly die soonest in lethal concentrations or inten- 

 sities of external agents and are inhibited most under conditions 

 so severe that acclimation does not occur. Both disintegration 

 processes and inhibition are differential, paralleling the polar 

 axis in early development and other axes that arise later. 



(b) All of the modifications, produced under different condi- 

 tions, are essentially similar, differing not in kind but in degree. 

 No evidence was found of "specificity," "blastolysis" or "dis- 

 organization " as effective factors in the production of any of the 

 modifications. 



(c~) The experimental data indicate that any type of abnor- 

 mality may be produced under the influence of any inhibiting 

 agent by controlling the concentration or intensity of action, the 

 length of exposure and the stage in development (physiological 

 condition) of the egg or embryo when exposed. 



4. Differential inhibitions, appearing under conditions that 

 prevent acclimation, are evidenced in the following and other 

 ways: 



(a) By the relatively slower division of the more active animal 

 pole cells with the result that the cell size ratio approaches and 

 may become equal to one. 



(6) By the retardation or prevention of downward movement 

 of material from the animal hemisphere, which takes place, 

 normally, most rapidly in the sagittal plane, but which under 

 these conditions, is most retarded in this plane. 



(c) By the retardation of the dorsal lip region with the forma- 

 tion of V-shaped blastopores and oval yolk plugs that may be 

 several times longer in the sagittal plane than broad. 



(d) By the retardation of both dorsal and lateral lips of the 

 blastopore resulting in the formation of flat-crescent blastopores 

 and a whole series of modifications between this and, 



(e) Perfect equatorial gastrulae. 



(/) By the appearance of a secondary invagination apical 

 to the original blastopore. 



